Optical recording medium comprising a cross-linked buffer layer

ABSTRACT

The invention relates to an optical recording medium, in particular to a compact disc, comprising the following layers: 
     a) a transparent substrate with a spiral having an indented or protruding geometry; superposed by 
     b) a recording layer; optionally superposed by 
     c) a protective coating, wherein the recording layer comprises a partially transparent layer; superposed by a transparent buffer layer comprising a cross-linked material; superposed by a thick layer, said layers forming together a Fabry-Perot etalon.

BACKGROUND OF THE INVENTION

The invention relates to an optical recording medium comprising across-linked buffer layer, a method for the manufacture thereof and anapparatus for the continuous manufacture of the same.

More particularly the invention relates to optical recording media suchas compact discs (CDs) and digital tapes or cards, usually known as WORMmedia (write-once-read-many-times) and rewritable CDs and tapes. Thesemedia allow digital information such as data, music, and image, to bewritten by the user.

In conventional read-only CDs the information is stored in pits embossedin the disc. The reading is based on diffraction of reflected light onthe regular pit structure. Interference of the diffracted orders isdependent on the position of the laser spot. This results in amodulation in reflection, which is used for reading the information. Theconventional read-only CDs are only suitable for large-scale productionas the production steps (for obtaining a written disc) are rathercomplicated and therefore only cost-effective in mass production. Hencethere is need for CDs and digital tapes or cards which can be producedin smaller quantities or can even be written by the user himself. InEP-A2-0353391 an optical recording medium is described comprising alight-transmitting substrate having a deformable surface, alight-absorptive layer overlaying the deformable surface, and alight-reflective layer overlaying the light-absorptive layer, saiddeformable surface being deformable by energy generated upon absorptionof the writing laser beam by the light-absorptive layer, to formoptically readable pits. The reading is again based on pit-edgeinterference. During irradiation with the reading laser the lighttravels through the light-absorptive layer and is reflected by thereflective layer. As the refractive index within a pit differs from therefractive index outside it (land), the optical path length within thepit differs from that of the land. The laser light which falls withinthe pit interferes with the light which falls on the land. The resultingreflection modulation at this pit/land edge is used for reading theinformation. There is still need for improvement in reflectivity andcontrast in the WORM media and rewritable media proposed so far. In ourco-pending PCT patent application WO 96/16402 an optical recordingmedium is described which comprises a substrate provided with apartially transparent thin reflective layer, which is provided with alayer comprising liquid-crystalline material having a thickness between100 and 1200 nm, which is provided with a thick layer having areflectance over 50%. Thus, in comparison with the above-describedoptical recording media, an extra thin reflective layer is present,resulting in the liquid-crystalline material layer being sandwichedbetween two reflective layers. In this way a Fabry-Perot interferometeris created. The Fabry-Perot phenomenon is used to obtain a difference inreflection between the written and the unwritten state in the digitalstorage medium. Although a satisfactory contrast can be obtained withthe concept described in application WO 96/16402, several problems stillhave to be solved. Some of these problems have been solved inco-pending, not yet published European patent application 95203502.0. Ithas now been found that substantial further improvements, in particularan improved writing contrast and an improved manufacturing process, canbe obtained by applying a transparent buffer layer comprising a materialwhich is capable of cross-linking, and an initiator which is preferablya photo-initiator.

SUMMARY OF THE INVENTION

In its most general concept the present invention relates to an opticalrecording medium comprising the following layers:

a) a transparent substrate with a spiral having an indented orprotruding geometry; superposed by

b) a recording layer; optionally superposed by

c) a protective coating, wherein the recording layer comprises apartially transparent layer; superposed by a transparent buffer layercomprising a cross-linked material; superposed by a thick layer, saidlayers forming together a Fabry-Perot etalon.

The invention further comprises an article comprising the followinglayers:

a) a transparent substrate with a spiral having an indented orprotruding geometry; superposed by

b) a partially transparent layer; superposed by

c) a transparent buffer layer comprising a material containing acompound capable of cross-linking and an initiator, optionally,superposed by

d) a thick metal layer, optionally, superposed by

e) a protective coating.

The present optical recording medium has many advantages over the knownoptical recording media and over those of the co-pending patentapplications. The optical recording medium can be manufactured in aneconomical manner, shows an improved thermal stability, and above allhas a surprisingly improved adhesion of the metal of the thick layer tothe buffer layer. Moreover, as in the co-pending patent applications,the metal of the thick layer is not necessarily gold, but also cheapermaterials as aluminum, silver, or alloys thereof can be used. Thecross-linked buffer layer further improves the pit integrity and thepits are therefore better defined than in corresponding opticalrecording mediums having non-cross-linked buffer layers, resulting inimproved writing properties.

By virtue of the partially transparent layer, the buffer layer and thethick layer a Fabry-Perot interferometer is created. Information can bewritten by deformation of one or more of the partially transparentlayer, the buffer layer, the thick layer, and the substrate. Thedeformation results in changes of the Fabry-Perot reflection, and adecrease or increase of reflection occurs. The tracking (keeping thewriting laser within the tracking means) can take place by employing thedifference in amplitude and/or phase of the partially transparentlayer/substrate interface within and outside the tracking means,resulting in diffraction, as will be explained in further detail below.

The term "transparent" as used in "transparent buffer layer" means thatsaid buffer layer is transparent to the wavelength of the laser used fortracking and reading the optical recording medium, i.e. not more thanabout 20% of the light is absorbed by the buffer layer.

The term "tracking means" denotes a spiral track in the substrate whichcan have the geometry of a groove (indented) or dike (protruding), or isthe groove image which is obtained in the buffer layer/thick layerinterface, or else is a combination of both.

The term "thick" in "thick layer" means a thickness sufficient toprovide 0 to 70%, and preferably 0 to 50% transmission of the incidentlight. In conventional CDs 0 to 5% transmission is preferred.

For convenience's sake, from now on the term CD will be used to refer toall optical recording media according to the invention, withoutrestriction.

A Fabry-Perot interferometer typically consists of two parallelreflecting layers placed at some distance from each other. Thereflectance of the Fabry-Perot interferometer as a function of thethickness of the buffer layer shows an oscillating behavior. Thereflectance minimum depends on thelayer thickness and the complexreflective index of the partially transparent layer. The thickness ofthe buffer layer is chosen in such a manner that the reflectance fromthe land area is about the maximum reflectance of the CD. In the presentinvention the Fabry-Perot interferometer results from the partiallytransparent layer, the buffer layer, and the thick layer, and anadditional mirror layer as is used in prior art writable CDs is notlonger necessary. It is required that the thickness of the recordinglayer does not exceed the focus depth of the light source used. Fordiode lasers as used in recording and playing media this is typicallyless than 5 μm.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is preferred for the recording medium according to the invention tobe in the high reflective state of Fabry-Perot with a reflectance above50% in the unwritten state from the groove or dike and a reflectanceabove 60% from the land, because conventional CD-players need about 20%background reflection from the groove or dike in the written state fortracking, and the CD-standard demands 60% writing contrast. OnceCD-players requiring smaller background reflection become available,lower reflectance than 50% can be employed. A reflection above 20% inthe unwritten state from the groove or dike and a reflectance above 25%from the land is then preferred. Upon writing the Fabry-Perot is detunedby deformation of one or more of the partially transparent layer, thebuffer layer, the thick layer, and the substrate, resulting in adecrease of the reflection by at least 10%, and preferably by at least40% of the unwritten state. For DVD-R media it is also possible to applya Fabry-Perot etalon tuned for low reflection from the groove in theunwritten state, which changes into a high reflective state afterrecording.

In the conventional read-only CDs the recorded information is stored ina spiral track in which regions of low reflectance (pits) are alternatedwith regions where the background reflectance (land) is higher than 60%.The pit length varies from 0.9 to 3.3 μm in 0.3 μm steps. In the longestpits (generating the 11T signal) the reflectance must drop to below 40%of the background reflectance. The read-out laser in a conventional CDplayer has a wavelength between 780 and 830 nm, in general 780±10 nm. Inorder to be compatible with the read-only CD, a CD according to theinvention should have a reflectance in the unwritten state of at least60%, and the reflectance in the longest pit should be below 40% of thebackground reflectance when using a conventional read-out laser for CDplayers. The present invention provides CDs having parameters which,after recording, can be set to make the CD compatible with theconventional read-only CD (hereinafter referred to as the CD standard).In the known DVD-type compact discs the information is stored in spiraltracks in which regions (pits) of lower and higher reflection arepresent. These regions can be read by using focused laser light of650±10 nm. The beginning and the end of each pit is recognized as arapid change of the reflectance. The present invention provides CDshaving parameters, which, after recording, can be set to make the CDcompatible with the known DVD-type compact discs.

The buffer layer comprises materials containing compounds capable ofcross-linking. Suitable compounds capable of cross-linking are known inthe art. Preferred compounds have functional groups, and are optionallymixed with mono- and/or multifunctional compounds to control therheology during processing and the cross-link density. Condensationmono-, oligo- and polymers and radically polymerizing mono-, oligo- andpolymers having functional groups capable of cross-linking can be used.The suitable compounds can form at least two bonds through theirfunctional groups. Examples are (di)acrylates, (di)epoxides, (di)vinylethers, and diols which can be converted into polymeric networks.Particularly interesting compounds are low-molecular weight monomers ofthe acrylic type, for instance such as disclosed by Lub et al., LiquidCrystals, 1995, Vol. 18 (2), 319-326, which is incorporated byreference. The skilled person will know how to choose other suitablecompounds which are known in the art. To the low-molecular weight mono-,oligo-, or prepolymers, for instance an acrylate or methacrylatemonomer, a few percent of an initiator (usually 1-5%), preferably aphoto-initiator such as Irgacure 651®, diphenyliodoniumhexafluoroarsenate, ferrocenium salts, or other known initiators, orcombinations thereof, are added. The photo-initiated polymerizationoccurs in situ and is performed by irradiation of the monomers during afew seconds to a few minutes, for instance by UV radiation (typically250-400 nm; 0.1-12 mW/cm²) or light (typically 400-500 nm). This processis well-known in the art, see for instance Broer et al., Macromolecules,1993 (26), 1244-1247 and Hikmet et al., Polymer, 1993, Vol. 34 (8),1736-1740, which are incorporated by reference. The initiator may alsobe a thermal initiator, for example, AIBN (azoisobutyronitrile) orTrigonox B® (di-tert-butylperoxide).

To apply the buffer layer, the low molecular weight material andoptionally other additives are preferably dissolved in a suitablesolvent and spin-coated. Additives are, for instance, adhesion promotingagents, exothermally or endothermally decomposing agents for controllingand improving the recording properties, and colorants for fine-tuning ofthe optical thickness of the buffer layer. The optical thickness isdefined by the product of the refractive index and the physicalthickness of the buffer layer. Further, recording light absorbing agentsmay be used as an additive for fine-tuning the heat effects duringrecording, which results in optimal pit dimension, integrity, andrecording sensitivity.

Other conventional means of applying coatings with accurate thicknessmay also be employed. Preferably the cross-linkable buffer layer isobtained by spin-coating. By selecting the right monomer viscosity andsolids content the desired layer thickness and layer quality areguaranteed. After spin-coating the buffer layer is heated or irradiatedto effect cross-linking of the material. By using a proper combinationof the spin-coating procedure and spin-coat solution and a specificallyselected track geometry in the substrate a groove image is obtainedwhich is optimal for tracking. The invention further comprises a methodof tracking using said groove image. The track geometry is the depth,height, and width of the groove or dike, comprising U- and V-shapedtracks. By using a cross-linkable buffer layer the simultaneousoptimization of the buffer layer's quality and thickness, and of thegroove image depth, is facilitated because more degrees of freedom havebecome available. The number of cross-linkable groups and the relativepercentage of initiator determine the cross-link density of the bufferlayer and thus the Tg and the modulus of the cross-linked buffer layer.These properties affect the amount and type of deformation that can beobtained during writing.

The optical recording medium according to the invention comprises asubstrate with a spiral having an indented or protruding geometry. Themedium is read through the substrate. Therefore, the substrate should beoptically transparent to the laser light used for reading and writing.In conventional CD players laser light with a wavelength of 780 nm isused. Suitable substrates which are optically transparent at thiswavelength and have sufficient thermal stability and resistance tohumidity are polycarbonates, amorphous polyolefins (APO),poly(methylmethacrylates) (PMMA), and glass. Polycarbonate substratesare preferred for their price and ease of handling. Further, theproperties of polycarbonate substrates are within the CD standard.Amorphous polyolefins also have properties which lie within the CDstandard, but these substrates are more expensive than polycarbonates.However, polycarbonate is susceptible to chemical attack by almost everysolvent normally used for applying the buffer layer material. With theCD according to the present invention this problem is circumventedbecause the partially transparent layer protects the substrate fromsolvent attack if this layer is thick enough. Thus, a broad choice ofsolvents can be used. Accordingly, in the case of the CD according tothe invention, polycarbonate can be readily used as a substrate. Forhigh density CDs the substrate has to be transparent in the wavelengtharea of 610 to 700 nm. For ultra-high density CDs, the substrate mustalso be transparent at wavelengths shorter than 610 nm.

As mentioned above, a substrate with a spiral having an indented orprotruding geometry is used for the CD according to the invention. Sucha tracking means preferably takes the form of a spiral-shaped groovewhich is molded in the substrate. This track is used to control thelaser spot position during reading and writing. As the partiallytransparent layer has an index of refraction which is different fromthat of the substrate, the tracking can be done via the interference anddiffraction caused by the difference in reflected amplitude or phasefrom that of the partially transparent layer/substrate interface withinand outside the groove. As regards the tracking the geometry of thetrack is decisive. Usually a track width of 0.1-1.2 μm, and preferablyof 0.3-0.6 μm, is used for conventional CDs. For recordable DVD media, atrack width of 0.15-0.45 μm is preferred. The track depth or height isan important parameter which has to be chosen in relation to thethickness of the buffer layer and the partially transparent layer, andis usually in the range of 20-400 nm. It was found that a relativelyshallow track depth of 50 nm in combination with a buffer layerthickness of 285 nm and an aluminum partially transparent layer of 7 nm,or in combination with a buffer layer thickness of 220 nm and a siliconpartially transparent layer of 50 nm, results in an optimal trackingcontrast while preserving the reflection properties. It is also possibleto obtain optimal tracking conditions with a track depth of more than100 nm.

In order to reduce the loss of laser light by reflection at theair/substrate interface, the substrate may be provided with ananti-reflection structure on the side not covered with the thick layer.

For the partially transparent layer both metals and non-metal materialscan be employed as long as the layer can be made thin enough to bepartially transparent to the laser light. This is usually in the rangeof 0.3-30 nm. The layer thickness of the partially transparent layer canbe expressed as a percentage of the light passing this layer as appliedonto the transparent substrate on one side, and with air on the otherside. The thickness of the partially transparent layer should preferablybe between 25% and 90% as measured with light having a wavelength aboutequal to the wavelength of the light used for recording or read-out ofthe CD. A substrate without a partially transparent layer should have anormalized transmission of 100%. Metals usually have a high imaginarypart of the index of refraction. This means that they are both highlyreflective when used in a transparent environment (such as air andpolycarbonate) and absorptive. Accordingly, upon writing with a laserthe laser light is reflected (tracking) and absorbed by the partiallytransparent metal layer. The absorbed laser light is converted intoheat, and deformation of one or more of the partially transparent layer,the buffer layer, the thick layer, and the substrate occurs. Suitablemetal materials for the partially transparent layer are aluminum,tantalum, gold, silver, nickel, iron, titanium, chromium, vanadium,nickel-gold, nickel-vanadium, nickel-chromium, and other alloys.Preferred are gold, aluminum, nickel, tantalum, vanadium, chromium, oralloys thereof. It is preferred that the metals used have a relativelylow heat conductivity, in order to preserve the pit integrity. Suitablenon-metal materials are, for instance, silicon, silicon nitride, silicongermanium, silica, SiO_(x), SiO-germanium, etc. This type of materialusually has a high real part of the index of refraction and a smallimaginary part. This means that the material is moderately reflective,but hardly absorptive. Upon writing with a laser on CDs with this typeof partially transparent layer, the laser light is reflected by thepartially transparent layer, but the absorption of the laser light willhave to take place in the buffer layer. Thus, when employing a partiallytransparent non-metal layer in the CD according to the invention or avery thin partially transparent metal layer (having more than 35%transmission), it is preferred to use an absorptive buffer layer or athick partially transparent non-metal layer, for instance thicker than30 nm. Especially preferred are germanium, silicon germanium alloys, andsilicon because of their high real parts of indices of refraction. Thepartially transparent layer may be applied onto the substrate by anyconventional method normally used in the field such as vacuumdeposition, electron beam deposition, and sputtering.

The thick layer is preferably a metal layer such as gold, aluminum,silver, copper, chromium, nickel, platinum, alloys such asaluminum-titanium, copper-aluminum, gold-aluminum, etc. superposed onthe buffer layer by, for instance, chemical vapor deposition, orsputtering. This thick layer should not be transparent to the laserlight. As aluminum and silver are cheaper than gold, and thereflectivity of an aluminum or silver layer with a thickness above 30 nmis sufficiently high, the use of aluminum, silver or alloys thereof forthe thick layer is preferred.

The protective coating can be any resin having a good impact resistance.Usually a UV curable resin is used, which is applied by spin-coating,followed by UV irradiation for curing. Other suitable materials for theprotective coating are epoxy resins, acrylate resins, silicone hard coatresins or urethane resins. The thickness of the protective coating isusually not critical and is usually within the range of 1 to 30 μm,preferably 5 to 15 μm.

If so desired, in order to enhance the deformation in the thick layer, adeformable layer, for instance a low-modulus organic polymer ofhigh-molecular weight, can be used between the thick layer and theprotective coating.

The invention is further directed to a method for the manufacture of anarticle according to the invention. Said method comprises the steps ofapplying a partially transparent layer onto a substrate with a spiralhaving an indented or protruding geometry, and applying onto saidpartially transparent layer a buffer layer comprising a materialcontaining a compound capable of cross-linking, and an initiator.

This article can be further manufactured into the optical recordingmedium according to this invention by above-mentioned method, followedby activation of the initiator in order to cross-link the transparentbuffer layer; applying onto said buffer layer a thick layer, andoptionally applying a protective coating onto said thick layer.

As mentioned-above, the partially transparent layer and thick layer canbe applied by, for instance, vacuum deposition, electron beamdeposition, or sputtering. The buffer layer can be applied by, forinstance, spin-coating, sputtering, and the like.

This method of manufacture can easily be made into a continuous process.An apparatus for the manufacture of conventional read-only CDs caneasily be adapted to the fabrication of the optical recording mediumaccording to the invention by inserting means for applying the partiallytransparent layer, as well as means for applying the buffer layer, intothe conventional line. The invention is also directed to an apparatusfor the continuous manufacture of an optical recording medium accordingto the invention comprising means for transporting a substrate with aspiral having an indented or protruding geometry (1), means for applyinga partially transparent layer onto said substrate (2), means forapplying a buffer layer comprising a compound capable of cross-linking,and an initiator onto said partially transparent layer (3), means forcuring the buffer layer (4), means for applying a thick layer onto saidbuffer layer (5), and optionally means for applying a protective coating(6). Said apparatus may further have guiding means between means (2) and(3), means (3) and (4), means (4) and (5), and means (5) and (6).

The invention is further illustrated by the following examples.

EXAMPLE 1

Difunctional aliphatic urethane acrylate (3.74 g) as a dilution in 20%TPGDA (tripropyleneglycol diacrylate) (Actilane® 200TP20; AkrosChemicals) and photo-initiator Irgacure 369 (83.5 mg; Ciba-Geigy;2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1) weredissolved in 21.4 ml of a mixture of 4-hydroxy-4-methyl-2-pentanone andn-butanol 9:1 (w/w), and the solution was filtered over a 0.2 μm filter.This solution was used in the following step (Example 2).

EXAMPLE 2

A thin aluminum film of 10 nm thickness was deposited on a 1.2 mm thickpolycarbonate substrate having a pregroove with a depth of 170 nm and awidth of 0.5 μm and a track pitch of 1.6 μm. On said thin aluminum layera buffer layer was spin-coated from a solution according to Example 1.The thickness of the buffer layer was 250 nm. If desired, the layer wasdried at 60° C. on a hot plate, and thereafter purged for 1 min withnitrogen. Under further purging with nitrogen the layer was irradiatedwith UV light (4.5 mW/cm² at 365 nm) for 1 min. Onto said buffer layer a100 nm thick aluminum layer was vacuum-deposited. After drying in avacuum oven at 40° C., a protective layer of UV curable epoxyacrylateresin was spin-coated thereon and cured. The resulting CD was evaluatedusing an evaluation equipment employing a laser beam of 780 nm. A signalto noise ratio (CNR) of 51 dB was obtained by 1.3 m/sec, 720 kHz and 8mW recording conditions, and a reflectivity of 72% was obtained in theland and a reflectivity of about 50% was obtained in the track with 0.7mW read-out power. The disc appeared to be replayable in a compact discplayer and the disc appeared to satisfy the specifications of the CDstandard.

EXAMPLE 3

The mixture of Example 1 was mixed in a 1:1 ratio with a solution ofpoly(methylmethacrylate) with an average Mwt of 4800 D in4-hydroxy-4-methyl-2-pentanone and n-butanol (9:1). 9.5 wt % of thissolution was spin-coated onto a CDR substrate with 185 nm deep groovesand a 400 nm groove width which was coated with an 8 nm thick aluminummetal layer. After spin-coating the organic layer was illuminated by alow-power mercury lamp (250 W, 4.5 mW/cm2 at 365 nm) for 1 minute tocure it.

After drying an 80 nm thick aluminum reflective layer was sputtered ontothe organic layer, and finally a protective layer (UV curableepoxyacrylate resin) was applied onto the reflective layer and alsocured.

The resulting CD-R was recorded using a commercially availableCD-R-recorder (Yamaha) and subsequently evaluated. A CNR value of morethan 47 dB was obtained, compliant with the CD standard.

When this disc was analyzed it was attempted to remove the protectiveand reflective layers by sticking an adhesive tape (Scotch) onto theprotective layer and subsequent pulling. The tape gave way from theprotective layer surface, the CD was unaffected.

COMPARATIVE EXAMPLE 4

A CD was made according to Example 3, except that the organic layerconsisted of pure PMMA, i.e. no cross-linkable agent was used.Evaluation of the disc showed bad jitter values and low (34 dB) CNRvalues.

The adhesive tape test (see example 3) resulted in the removal of thecombined reflective and protective layer.

EXAMPLE 5

A spin-coat solution was made consisting of a 20 wt % of Actilane200TP20 (see Example 1), and 10 wt % of the dye Victoria Blue R(Aldrich) in 4-hydroxy-4-methyl-2-pentanone. The disc was made accordingto the procedure described in Example 3. Recording resulted in opticallyreadable pits.

EXAMPLE 6

A CD-R was made according to the method described in Example 3, themixture further containing 20 wt % of dianolacrylate (DA 121, AkzoResins), the total solids content of the spin-coat solution being 13 wt%. The method also differed from Example 2 in that the substrate had 185nm deep grooves instead of 230 nm deep grooves.

The disc obtained was recorded using a commercial CD-R recorder(Yamaha). Evaluation showed optically readable pits. The writingcontrast (reflection before and after recording) was of the oppositesign of that of conventional CD-Rs, i.e. the reflection after recordingwas higher than before.

EXAMPLE 7

A CD-R was made according to the method described in Example 3,differing in that the solution described in Example 1 contained 30 wt %of TPGDA instead of 20 wt % (TMPTA: trimethylolpropanetriacetate, AkzoResins). The disc further differed from Example 3 in that the substrateused had 70 nm deep grooves instead of 185 nm deep grooves.

The disc was recorded using a commercial CD-R recorder (Yamaha).Evaluation showed optically readable pits and reverse writing contrast(as explained in Example 6).

EXAMPLE 8

A CD-R was made according to the method described Example 7 using 10 wt% of TMPTA instead of 30 wt %. Evaluation of the recorded disc showedoptically readable pits and reverse writing contrast.

What is claimed is:
 1. An optical recording medium comprising thefollowing layers:a) a transparent substrate with a spiral having anindented or protruding geometry; superposed by b) a recording layer;optionally superposed by c) a protective coating, wherein the recordinglayer comprises a partially transparent layer; superposed by atransparent buffer layer comprising a cross-linked material; superposedby a thick layer, said layers forming together a Fabry-Perot etalon. 2.The optical recording medium of claim 1 wherein the partiallytransparent layer comprises aluminum, nickel, tantalum, or alloysthereof.
 3. The optical recording medium of claim 2 wherein the thicklayer comprises aluminum, silver, gold, copper, or alloys thereof. 4.The optical recording medium of claim 2 wherein the substrate comprisespolycarbonate.
 5. An article suitable for the manufacture of the opticalrecording medium of claim 2 comprising the following layers:a) atransparent substrate with a spiral having an indented or protrudinggeometry; superposed by b) a partially transparent layer; superposed byc) a transparent buffer layer comprising a material containing acompound capable of cross-linking, and an indiator.
 6. The opticalrecording medium of claim 1 wherein the thick layer comprises aluminum,silver, gold, copper, or alloys thereof.
 7. The optical recording mediumof claim 6 wherein the substrate comprises polycarbonate.
 8. An articlesuitable for the manufacture of the optical recording medium of claim 6comprising the following layers:a) a transparent substrate with a spiralhaving an indented or protruding geometry; superposed by b) a partiallytransparent layer; superposed by c) a transparent buffer layercomprising a material containing a compound capable of cross-linking,and an initiator.
 9. The optical recording medium of claim 1 wherein thesubstrate comprises polycarbonate.
 10. An article suitable for themanufacture of the optical recording medium of claim 9 comprising thefollowing layers:a) a transparent substrate with a spiral having anindented or protruding geometry; superposed by b) a partiallytransparent layer; superposed by c) a transparent buffer layercomprising a material containing a compound capable of cross-linking,and an initiator.
 11. An article suitable for the manufacture of theoptical recording medium of claim 1 comprising the following layers:a) atransparent substrate with a spiral having an indented or protrudinggeometry; superposed by b) a partially transparent layer; superposed byc) a transparent buffer layer comprising a material containing acompound capable of cross-linking, and an initiator.
 12. The article ofclaim 11 wherein the transparent buffer layer comprises one or moremonomers or oligomers and an initiator.
 13. The article of claim 12wherein the monomer is of the acrylic type.
 14. The article of claim 12wherein the initiator is a photo initiator.
 15. The article of claim 11wherein the initiator is a photo-initiator.
 16. The article of claim 11,further comprising one or more additives selected from an adhesionpromoting agent, an exothermally or endothermally decomposing agent, anda recording light absorbing agent.
 17. A method for the manufacture ofan article according to claim 11 comprising the steps of:applying thepartially transparent layer onto the substrate with a spiral having anindented or protruding geometry; and applying onto said partiallytransparent layer the buffer layer comprising a material containing acompound capable of cross-linking, and an initiator.
 18. A method forthe manufacture of an optical recording medium according to claim 10,further comprising the steps of followed by activation of the initiatorin order to cross-link the transparent buffer layer; applying onto saidbuffer layer a thick layer, and optionally applying a protective coatingonto said thick layer.
 19. An apparatus for the continuous manufactureof an optical recording medium according to claim 1 comprising means fortransporting a substrate with a spiral having an indented or protrudinggeometry (1), means for applying a partially transparent layercomprising a compound capable of cross-linkage, and an initiator ontosaid substrate (2), means for applying a buffer layer onto saidpartially transparent layer (3), means for curing the buffer layer (4),means for applying a thick layer onto said buffer layer (5).
 20. Theapparatus of claim 19 further comprising means for applying a protectivecoating onto the thick layer.